I think the depreciation of the IS-F Is on par with the Jag XFR....can get early models of both for mid teens now whereas the equivalent C63 or M3 is a few grand more.
A lot will depend on mileage and service history. A higher mileage E46 M3 can be had for under £10k, so I expect the IS-F will drop to this level at some point before creeping back up as the numbers fall, the lower mileage/full main dealer history M3's are holding low to mid teens so the equivalent IS-F should too.
Its a rarer car than the others which is a plus, but also doesn't get the same coverage so goes under the radar for potential C63 and M3 buyers.
If you look at models such as the Clio V6, E36 M3 and Impreza P1, they have doubled, even tripled, in price over the past decade. I think cars such as the IS-F, Monaro, RS4 etc will eventually follow suit.